while 

syntax:	while (expression)	A condition that is true or false
		   stmt			Body.  {} compound statement
semantics: Evaluate expression, if true execute the statements of the body and then re-execute the while statement.
If condition is true, execute the body and then loop back up and if the condition is (still) true, execute the body and then loop back up...
When the condition is false, skip the body, the while is finished.
Loop while, or as long as, the condition is true.
Loop until the condition is false.
Repeatedly execute some code (the body) as long as some condition holds, is met, is true.
Condition should eventually be false, else have an infinite loop (will loop forever). e.g. while (true) or while (1) 
//read numbers from user, output the square root.  0 to quit
cout << "Enter nos. for sqr. rooting.  0 to quit: ";
cin >> num;      //first number user enters.  "priming" read
while (num != 0) {       //until number entered is a 0
  cout << sqrt(num) << endl;
  cin >> num;            //next number user enters
}

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//same, but check for negative numbers
cout << "Enter nos. for sqr. rooting.  0 to quit: ";
cin >> num;      
while (num != 0) {     
  if (num > 0)   //body of any statement can contain any statements
    cout << sqrt(num) << endl;
  else
    cout << "No square root of negative number.";  
  cin >> num;              
}

-------------------------------------------------------

//input 5 numbers from user and sum them
sum = 0;         //initialize the sum to 0
count = 0;       //count of number of numbers read
while (count < 5) { //loop five times
  cin >> num;
  sum += num;    //add num to sum; short for   sum = sum + num
  count++;       //increment counter of numbers
}
cout << "Sum is " << sum;

-------------------------------------------------------

//user tells how many numbers to input and sum
cout << "Enter number of numbers: ";
cin >> num_nums;
sum = 0;
count = 0;
while (count < num_nums) {          //loop num_nums times
  cin >> num;
  sum += num;
  count++;
}
cout << "Sum is " << sum;

-------------------------------------------------------


//input numbers from user.  Negative number indicates end.
sum = 0;                  //no need for count variable
cin >> num;               //first number from user
while (num >= 0) {        //until a negative number is entered
  sum += num;
  cin >> num;
}
cout << "Sum is " << sum;
if the first number entered is negative, the condition is false the first time in the while statement, thus the body wouldn't be executed even once.

A while loop will execute 0 or more times. 



//determine the number of digits in an integer
//Ex. 548 has 3 digits, 2974 has 4 digits
cout << "Enter an integer: ";
cin >> num;
digits = 0;
while (num > 0) {
  num = num / 10;          //integer divide by 10
  digits++;
}
cout << "Number of digits is " << digits;

Trace this code with some examples.  Show the values of the variables
that result from execution of each statement.  Does it work with
negative number?  How to correct it?

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//read chars until #, 
//count numbers of lowercase letters and of uppercase letters
//and of all other chars

int lowers=0, uppers=0, others=0;
char c;

cin >> c;
while (c != '#') {    //until # is entered
  if (islower(c))     //c is one of 3 categories
    lowers++;
  else if (isupper(c))
    uppers++;
  else
    others++;
  cin >> c;
}
cout << "Number of lower letters: " << lowers << endl;
cout << "Number of upper letters: " << uppers << endl;
cout << "Number of all other chars: " << others << endl;

Files

Basic file I/O example program file.cpp
An input file you can use with the file.cpp program.

Next (logical operators)

©David Wills